Gnash only supports Flash up to version 7 (and parts of version 9) while the official Flash version is currently v.10!
It also lacks Lightspark’s ability to use hardware acceleration, which is a feature I’ve been waiting for very long.
Just think about it. Eye-popping, graphic intense games playable directly inside the browser: http://www.youtube.com/watch?v=6TKGNS1N1yo

I tried Gnash out, back in my Linux days on Ubuntu, and almost no Flash-application I tried worked with it.

And no, I didn’t try Lightspark yet since there is no installable binary yet. Only the sources can be downloaded and to be honst: I currently don’t feel like compiling this myself :-)